1. Field of the Invention
This application relates to power line management. More particularly, the present arrangement relates to thermal line ratings.
2. Description of Related Art
The thermal rating of an overhead power line is the maximum current that the line can transfer without overheating. The line rating is a function of, among other things, the weather conditions seen along the line, including wind speed, wind direction, air temperature, and solar heating.
The thermal rating of transmission lines is based on the conductor sag. As electrical current increases through an overhead conductor, the conductor temperature increases and therefore the conductor expands and sags. Each line has a minimum clearance to ground, which should never be violated for safety reasons. The thermal rating represents the maximum current that may pass through the line so that the conductor will not sag below the minimum clearance. Any additional current would result in a safety violation because of excessive sag.
In general, an equation to calculate conductor rating is developed by first recognizing that the total input heat (per unit length) to a conductor must equal the total output heat in the steady state. The conductor is heated by ohmic losses (I2R) and solar input, and it is cooled by convection (wind) and radiation.
In simplified form—Irating=√((Qconvection+Qradiation−Qsolar)/R)                Qconvection=heat output from air temp, wind speed, etc. . . .        Qradiation=heat output from radiation to atmosphere        Qsolar=heat input from solar radiation        R=Resistance to electricity flowing through the line        
In the United States, the standard method for calculation of transmission line ampacity [rating] in steady and transient states is the IEEE Standard 738 (2006). Elsewhere in the world, the most commonly used standard is that established by Cigré (1992, 1997, 1999). The present arrangement describes a system using IEEE 738 for its rating method, however any rating method could be substituted as needed.
Most utilities have adopted a static (fixed) rating for each line based on a semi-worst-case weather approach. The most common weather assumptions made under that approach are 40° C. (104° F.) air temperature, full solar radiation (no clouds), and 0.6 m/s (2 ft/s) wind speed perpendicular to the conductor. At any given time, the actual line rating normally exceeds the static rating due to weather conditions being more favorable than assumed. A small percentage of the time, typically about 2%, the actual weather conditions are worse than those assumed resulting in an actual rating below the static rating. Therefore, using a static rating approach can result in either underutilizing much of the line's transfer capacity most of the time, or in violating the maximum permitted sag some of the time.
Currently, there are several methods for generating a dynamic (real-time) line rating for any given power transmission line. One such method is to use direct meteorological data obtained from weather stations installed along the power transmission lines. This allows for the use of actual weather data instead of assumed weather data. However, there is a high cost associated with placing the necessarily numerous weather stations required to collect an adequate amount of data to capture the spatial variability of wind along the transmission line, as noted for example in Cigré Technical Brochure 299 “Guide for selection of weather parameters for bare overhead conductor ratings.” In addition, weather station only based monitoring systems provide no direct feedback from the transmission line to verify that clearances are being maintained.
Another method for generating line ratings is to directly measure tension and/or sag using instruments placed periodically along the transmission line. This obviously produces direct feedback about the line conditions and thus is more accurate and reliable than the use of weather stations alone.
This direct tension/sag measurement method also requires an initial capital input, albeit less than that required for weather stations, to place the monitors. Also, these monitors work best when conductor temperatures are elevated and line sag is greater (caused by moderate to high line current and/or low wind speed conditions), which means they are good at detecting and mitigating safety concerns. When conditions are such that the line current is low or the wind speeds are very high, the lines do not heat up and sag as much, and the monitors become less effective at calculating line ratings. Under such conditions prior art systems using tension/sag monitors must default to alternate rating methods in order to generate a rating. These alternate methods again employ fixed assumptions for wind speed input, while using measured temperature values to determine the ambient and solar heat inputs. While operationally safe, these methods, as with the other prior art methods, understate the actual rating in most cases, and generate large step discontinuities in rating output when switching between the default method and the primary tension/sag based calculation.